The present invention relates to a sequential thermal cracking process for the thermal cracking of a hydrocarbon feedstock in a cascade of cracking units where the hydrocarbon feedstock is heated in a furnace to a predetermined maximum temperature and then thermally cracked in the cascade of cracking units.
EP 0 005 643 relates to a process of converting petroleum residuum to distillate products and premium coke. A heavy liquid hydrocarbonaceous material having an initial boiling point above 340° C. is combined with a hydrogen donor diluent and fed to a cracking furnace, operating at a temperature of from 480 to 540° C. and a pressure of 10.5 to 70 kg/cm2. The furnace effluent passes to a fractionator, where gases and distillates are taken off the upper section through lines, and a gas-oil fraction is taken off the mid portion of the fractionator, combined with hydrogen, and hydrogenated in a catalytic hydrotreater for reuse as hydrogen donor diluent. A portion of the hydrotreated gas-oil from the hydrotreater 14 is combined with the pitch fraction boiling above 510° C. from the bottom of the fractionator, and passed to a coker furnace where it is heated to coking temperature. The coker furnace effluent is then passed to a delayed coke drum for formation of premium coke. Vapors from the coke drum are returned to the fractionator, and coke is withdrawn from the bottom of coke drum. This document also teaches the addition of a second stage cracking furnace and a flash to remove light ends from the coker feedstock, wherein a first portion of the hydrogen donor diluent, after passing through the hydrotreater, is fed to the second stage cracking furnace, and a second portion is fed to the coker furnace.
U.S. Pat. No. 1,958,959 relates to the cracking of hydrocarbon oils for the production of lower boiling products such as gasoline or naphtha distillates, comprising passing fresh clean charging oil through a heating zone wherein it is raised to a cracking temperature under pressure, subjecting the resulting hot oil in a primary cracking zone to cracking temperatures at a superatmospheric pressure to effect cracking and vaporization, subjecting evolved vapors to fractionation to form a vapor fraction of light distillate and a higher boiling point condensate, withdrawing the condensate from the fractionating zone, passing condensate so withdrawn to heat the oil to a cracking temperature higher than that obtaining in the primary cracking zone, directing the oil thus heated into an enlarged digestion zone and maintaining the oil therein at a substantially constant temperature higher than that obtaining in the primary cracking zone and under higher pressures to effect cracking and digestion, expanding the cracked products from the enlarged zone into the primary cracking zone for distillation and preventing residual products of cracking from entering the heating zone.
GB 2 138 840 relates to a process for thermally cracking a heavy hydrocarbon oil, comprising the steps of: (a) feeding the heavy hydrocarbon oil into a first thermal cracking zone for thermally cracking the heavy hydrocarbon oil and for obtaining a first, thermally cracked product; (b) introducing the first product into a second thermal cracking zone for thermally cracking the first product and for obtaining a second, thermally cracked product and a pitch product, the second cracking zone having a plurality of cracking reactors which are connected in series, through which is successively passed the first product and to each of which is supplied a gaseous heat transfer medium to maintain the liquid phase therein, including the first product, at a temperature sufficient for effecting the thermal cracking and to strip the resulting distillable, cracked components from the liquid phase, the thermal cracking temperature in one reactor being so controlled as to become higher than that in its adjacent upstream-side reactor, the distillable, cracked components in respective reactors being removed overhead therefrom as the second product, the liquid phase in the downstream-end reactor being discharged therefrom for recovery as the pitch product; (c) separating the second product into a heavy fraction and a light fraction, (d) recovering the light fraction as a light product oil, (e) introducing the heavy fraction into a third thermal cracking zone for thermally cracking same and for obtaining a tar-containing product; and (f) recycling the tar-containing product to at least one of the reactors of the second thermal cracking zone. In the second thermal cracking zone the thermal cracking in the first cracking reactor is performed at a temperature of between 400 and 420 degr. C., that in the second reactor is between 410 and 430 degr. C. and that in the third reactor is between 420 and 440 degr. C.
U.S. Pat. No. 3,245,900 relates to a hydrocarbon conversion process wherein a reduced crude oil feed is supplied to vacuum distillation column, wherein the light gas oil is passed to hydrocracking zone, gasoline and lighter fractions comprising C4-hydrocarbons are withdrawn from the system, through line 8, the heavy gas oil is passed to a catalytic cracking zone, light cycle oil is passed to a hydrocracking zone, heavy cycle oil is passed to hydrocracking zone. Residuum is passed to solvent deasphalting, wherein the deasphalted oil is passed to a hydrocracking zone, to a coking zone, or to a thermal cracking zone.
US Patent application No 2012/298552 discloses a delayed coking process for the thermal cracking of whole crude oil in a delayed coking unit, where the whole crude oil feed stream is heated in a furnace and introduced into the delayed coking unit, wherein the gaseous and liquid product stream from the delayed coking unit are passed to a delayed coking unit fractionating column for recovering as separate side streams from the fractionating column naphtha, light gas oil and heavy gas oil, and recycling a portion of the heavy gas oil and reintroducing it with the coking unit product stream into the fractionating column. At least a portion of the fractionating column bottoms is mixed with the whole crude oil feed stream to form a mixed feed stream and introduced into the furnace.
Delayed coking is a thermal cracking process used in petroleum refineries to upgrade and convert petroleum residuum, which are typically the bottoms from the atmospheric and vacuum distillation of crude oil, into liquid and gas product streams leaving behind petroleum coke as a solid concentrated carbon material. A fired heater or furnace, e.g., of the horizontal tube type, is used in the process to reach thermal cracking temperatures of 485[deg.] C. to 505[deg.] C. With a short residence time in the furnace tubes, coking of the feed material is thereby “delayed” until it is discharged into large coking drums downstream of the heater.
In the practice of the delayed coking process, hydrocarbon oil is heated to a coking temperature in a furnace or other heating device and the heated oil is introduced into a coking drum to produce a vapour phase product, which also forms liquid hydrocarbons, and coke. The drum can be decoked by hydraulic means or by mechanical means. In most configurations of the delayed coking process, the fresh hydrocarbonaceous feed to the coking unit is first introduced into a coker product fractionating column, or fractionator, usually for heat exchange purposes, where it combines with the heavy coker oil products that are recycled as bottoms to the coking unit heater.
In a continuous process like fluid coking the coking reaction takes place in a fluidized coke-bed reactor (450-500 C), while part of the newly formed coke from the reactor is continuously withdrawn and heated in a separate heater vessel with air (500-600 C). This is done for heat balancing the unit and maintaining reactor temperature.
The present inventors assume that the cracking reactions take place at a fixed temperature that are high enough for coke precursors like diolefins to be formed which would in turn require more severe downstream upgrading for conversion to useful middle distillates. In addition, the coking yields from these processes are expected to be high because of the competing side reactions that convert saturates into coke precursors like diolefins and accelerate coke formation from these precursors.